What Are The Differences Between Cartilaginous Fishes And Bony Fishes?

The primary difference between cartilaginous and bony fishes is that the latter has a skeleton made of bone while the former has a cartilaginous skeleton.

A red tail catfish. Catfish are bony fish.

What Are Bony Fish?

Bony fish, also known as Osteichthyes, is a group of fish that is characterized by the presence of bone tissue. The majority of the fish in the world belong to this taxonomic order, which consists of 45 orders, 435 families, and around 28,000 species. This class of fish is divided into two subgroups: Actinopterygii (ray-finned) and Sarcopterygii (lobe-finned).

What Are Cartilaginous Fish?

Cartilaginous fish, also known as Chondrichthyes, is a group of fish that is characterized by the presence of cartilage tissue rather than bone tissue. This class of fish is divided into two subgroups: Elasmobranchii and Holocephali. Common names of cartilaginous fish include sharks, skates, sawfish, rays, and chimaeras.

Differences Between Bony Fish And Cartilaginous Fish

The principal difference between bony fish and cartilaginous fish is in the skeleton makeup. As previously mentioned, bony fish have a bone skeleton whereas cartilaginous fish have a skeleton made of cartilage. There are, however, several other differences between these two classes of fish. These differences are listed below.

Habitat

The vast majority of cartilaginous fish survive in marine, or saltwater, habitats. These fish can be found throughout the world’s seas and oceans. Bony fish, in contrast, are found in both saltwater and freshwater habitats.

Gills

Fish gills are tissues located on the either side of the throat. These tissues ions and water into the fish’s system, where oxygen from the water and carbon dioxide from the fish are exchanged. In other words, fish gills act as lungs. In bony fish, the gills are covered by an external flap of skin, known as the operculum. In cartilaginous fish, the gills are exposed and not protected by any external skin. The majority of fish, whether bony or cartilaginous, have five pairs of gills.

Reproduction

Bony and cartilaginous fish are also different in their reproductive behaviors. Bony fish reproduce in what is considered a primitive form of reproduction. These fish produce a large number of small eggs with very little yolk. These eggs are released into the open waters, among rocks on the river or seabed. Male fish then swim over the laid eggs, fertilizing them with sperm which may or may not reach all of the eggs. The eggs hatch into larvae, which are essentially defenseless. The larvae must then develop in the wild, where they are vulnerable to external threats. In this method, the survival rate is low.

In cartilaginous fish, reproduction occurs internally. The sperm is deposited inside of the female in order to fertilize a small number of large sized eggs with a significant amount of yolk. Cartilaginous fish embryo may develop in one of two manners. In one, the embryo develops within a laid egg, relying on the large yolk for nutrients. In the second, more advanced manner, the embryo are able to develop in the secure and protected environment of the mother’s uterus. These fish are born as fully functional organisms, rather than as developing larvae. After delivery or hatching, baby cartilaginous fish are able to hunt and hide from predators. This development process ensures a higher rate of survival.

Heart And Blood

In both classes of fish, the heart is divided into 4 chambers. In the hearts of cartilaginous fish, one of these chambers is known as the conus arteriosus, a special contracting heart muscle. In place of this chamber, bony fish have a bulbous arteriosus, a non-contracting muscle.

Another difference between the bony and cartilaginous fish is in how each class produces red blood cells. In bony fish, the red blood cells are produced in the bone marrow, the central part of the bone. This process is known as hemopoiesis. Cartilaginous fish lack bone marrow for hemopoiesis. Instead, these fish produce red blood cells in the spleen and thymus organs.

Jaw Structure

The jaw makes up part of the mouth, allowing it to open and close to grasp and digest food. Its structure is different in cartilaginous and bony fish. For example, bony fish have two sets of jaws: the oral jaw and the pharyngeal jaw. The oral jaw allows bony fish to catch food, to bite it, and to chew it. Teeth typically only grow along one side of the jaw. The pharyngeal jaw, located in the throat, further digests food by processing it before it moves from the mouth to the stomach.

In contrast, cartilaginous fish lack the pharyngeal jaw. The oral jaw of these fish is comprised of cartilage and separated into an upper and lower section. Each section is able to hold a number of teeth, which grow in multiple sets. Cartilaginous fish are even able to regrow teeth as they wear down over time.

Digestive System

The digestive system between bony and cartilaginous fish is also different. The intestine of cartilaginous fish is typically shorter than that found in bony fish, however, it spirals around internally to create a larger surface area that optimizes the absorption of nutrients. In bony fish, the intestine is longer and has no spiral shape.

J-shaped stomachs can be found in cartilaginous fish, while bony fish have a wide variety of stomach shapes and in some cases, no stomach at all.

The cloaca, the opening through which urine and feces is excreted is also different. It can only be found in cartilaginous and lobe-finned fish. In other bony fish, the urinary tract, genitals, and anus each have a separate opening.

Neutral Buoyancy

Fish must have an internal buoyancy system in order to prevent them from floating to the top of the water or from sinking to the bottom, known as neutral buoyancy. Bony fish are able to maintain neutral buoyancy with help from the swim bladder. The swim bladder is typically a two-sac organ that controls the volume of internal gasses to help the fish maintain a certain position in the water. This allows them to conserve energy they might otherwise use by swimming to maintain neutral buoyancy. Some bony fish have lost the swim bladder through evolution; most of these are bottom-dwelling species.

Cartilaginous fish are able to achieve neutral buoyancy due to the lighter weight of their cartilaginous skeleton and their more hydrodynamic exteriors. Some cartilaginous fish, like sharks, even swim to the surface of the water to take in air that helps them maintain their position in the water.